Atom bomb is based on the principle of uncontrolled fission chain reaction. Natural uranium consists of 99.28% of U238 and 0.72% of U235. U238 is fissionable only by fast neutrons. Hence, it is essential in a bomb that either U235 or Pu239 should be used, because they are fissionable by neutrons of all energies.
An atom bomb consists of two hemispheres of U235 (or 94Pu239), each smaller than the critical size and are kept apart by a separator aperture (Fig).
When the bomb has to be exploded, a third well fitting cylinder of U235 (or 94Pu239) whose mass is also less than the critical mass, is propelled so that it fuses together with the other two pieces. Now the total quantity is greater than the critical mass and an uncontrolled chain reaction takes place resulting in a terrific explosion.
The explosion of an atom bomb releases tremendous amount of energy in the form of heat, light and radiation. Temperature of millions of degree celsius and pressure of millions of atmosphere are produced. Such explosions produce shock waves. The release of dangerously radioactive γ−rays, neutrons and radioactive materials produce a health hazards over the surroundings for a long time. These bombs were used in world war II and were exploded over Hiroshima and Nagasaki in Japan. We are quite familiar with the amount of destruction and hazard.
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